Anaerobic acrylic adhesive formulations are mixtures based on acrylic esters capable of being polymerized by a radical route and consisting predominantly of multiacrylic esters and chemical generators of free radicals chosen from peroxy compounds, the said mixtures having a cure which is inhibited by oxygen.
These anaerobic formulations remain in the uncured state as long as they are in contact with atmospheric oxygen. On the other hand, when they are placed between metal surfaces or other surfaces which are impervious to atmospheric oxygen, the absence of contact with oxygen leads rapidly to their curing.
Anaerobic adhesive formulations of this kind are employed in various fields of industry, for producing seals or assemblies and especially for locking nuts.
Despite their growing use, anaerobic acrylic adhesive formulations still exhibit certain disadvantages. First of all, their chemical stability still remains insufficient because of the fact that, in the majority of cases it is necessary to incorporate latent accelerators of radical polymerization in these formulations in order to increase the rate of polymerization of the acrylic monomers once the said polymerization has been initiated by the free-radical generator. The presence of these ingredients in the anaerobic formulation can result in an undesirable curing of the said formulation before its use and, for example, during its storage. The incorporation of polymerization inhibitors, for example of compounds of the hydroquinone type, in the formulation reduces this risk of premature curing, but does not enable it to be eliminated altogether.
In addition, since the acrylic monomers present in the anaerobic formulations are predominantly multiacrylic esters whose polymerization produces a very compact polymeric network, the adhesive joints obtained after curing the said formulations exhibit an excessive mechanical rigidity and hence an inadequate resistance to impact and to peeling forces. Furthermore, anaerobic adhesive formulations have a thermal stability which is not satisfactory in most cases. Finally, the said formulations are highly fluid and this rules out the possibility of employing them for applications requiring the use of highly thickened adhesive formulations, especially adhesive bonding of large flat metal surfaces.
The objective of the invention is to provide a single-component anaerobic acrylic adhesive formulation which makes it possible to overcome the disadvantages of the corresponding earlier formulations.